I. FIELD OF THE INVENTION
This invention relates to diamond drag bits.
More particularly, this invention relates to diamond cutting elements for diamond drag bits.
II. DESCRIPTION OF THE PRIOR ART
Polycrystalline diamond compacts (PDC) are used extensively for cutters on drag bits for drilling soft to medium earthen formations in petroleum and mining exploration.
One of the most common type PDC cutters used in diamond drag bits for drilling predominately ductile medium strength formations is a cylinder type. A cylinder type PDC comprises a right cylinder tungsten carbide body with a thin layer (approximately 0.030" to 0.040") of polycrystalline diamond chemically and metallurgically bonded to an end face of the cylinder using a high pressure/high temperature (HP/HT) sintering process.
Although cylindrical PDC type cutters serve a very useful purpose in drilling, there are disadvantages in their application. Typically, a cylinder type PDC cutter is fixedly mounted, by brazing, in a socket formed on the outer surface of a blade fabricated on the drilling face of a drag bit. The diamond face of the cutter is oriented substantially parallel to a radius of the borehole being drilled. The PDC cutter is positioned with back rake and heel clearance for the diamond cutting face by tilting the trailing end of the cutter body upward in relation to the borehole bottom.
For drilling many ductile rock formations, presently used PDC cylinder cutters do not have the necessary clearance from the diamond cutting edge to the supporting blade outer surface paralleling the formation bottom. Therefore, the displaced rock formation interferes with the aforementioned blade outer surface and greatly retards the drilling rate.
Also, because of the limited stand-off of the current diamond cutters from the blade surface, sufficient cooling and cleaning of the cutter often is not accomplished. This is because the entire exposed portion of the cutter is indented into the ductile rock leaving no room for the drilling fluid to flush across the cutter face.
Because of the relatively small exposure of the diamond cutting face of the prior art cutters, the drilling life of the bit is limited to the amount of wear the cutter can experience before the rock formation continuously bears on the insert supporting blade outer surface stopping the drilling process. This wear amount is normally somewhat less than one-half the cutter diameter.
Normally, prior art cylindrical PDC cutters only have approximately one half of the cutter body surface area brazed into the socket on the blade surface. In cases where the rock formations are tough in shear and high impact loads are experienced, the braze strength is often insufficient to keep the cutters in place, thereby contributing to the termination of the bit run.
As a normal PDC cylinder type cutter wears during drilling, an ever enlarging wear flat forms on the bottom side of the carbide cylinder body on the trailing side of the diamond layer, thus slowing the drilling rate. The possible magnitude of the wear flat is determined by the original amount of heel clearance between the diamond cutting point and the blade outer surface.
A new PDC cutter for a drag type drilling bit is disclosed which overcomes the inadequacies of the prior art. The new asymmetric cutter provides more extension of the diamond cutting edge below the face of the drill bit. This permits better cleaning and cooling of the cutters and prevents the rock being drilled from bearing on the bit body surface, thereby significantly increasing the drilling rate and useful bit life.